Standardize login page card and sign-in buttons with Bootstrap styling

[TobyCyan]

The login page currently uses styling that is inconsistent with nearby pages such as the instructor welcome page.

We should update the login method container to use the same Bootstrap card structure used elsewhere, and simplify the login method buttons to use standard Bootstrap button classes.

[AkashBonagiri]

Hi! I'd like to work on this issue as my first contribution to TEAMMATES. I've set up the dev environment and can see the login page locally. Could you point me to any relevant style guide or existing Bootstrap patterns used elsewhere in the app I should follow for consistency? Happy to share a screenshot/approach before opening a PR if that helps.